Can mutex-locking function be marked as const

Question

I have thread-safe Document class representing custom document. It have getters (const functions) and setters to modify it's state. All these functions are mutex-protected to guarantee document will not changed until the method will be completely executed.

But due to QMutex usage, i can't mark state-accessed function as const without mutable usage. Capturing QMutex change it's state.

Is this code correct, or it can be written in a more nice way? Without hacky mutable usage.

class Document
{
    // This method should be const: it changes only mutex
    // and don't touch document state
    bool IsCorrect() const;
    ...
    mutable QMutex m_lock;
};

bool Document::IsCorrect() const
{
    // Capturing mutex object change it!
    QMutexLocker lock( &m_lock );
    ... Const-aware code
    bool result = ( m_context != NULL );
    ...
    return result;
}

Answer 1

Herb Sutter has an excellent talk on Channel9 about this topic.

The point relevant here is that in C++11, mutable has gained a new meaning regarding thread safety: "this data member is internally synchronised." That's precisely what a mutex is. Herb even says in that talk when talking about a mutex: "it wants to be mutable."

So having a mutable mutex and accessing it from a const member function is not hacky at all, and it's in fact one of the primary intended uses for mutable.